1. Field of the Invention
The present invention relates to a method and circuit arrangement for the sensor-less detection of the rotational angle of a damper-less synchronous machine, that is preferably excited by a permanent magnet and supplied by a rectifier.
2. Description of the Related Art
Because of progress in the areas of magnetic materials, power electronics and information electronics, synchronous machines excited by a permanent magnet are increasingly achieving importance in areas such as propulsion technology. They are distinguished over asynchronous machines in that they utilize simpler technical control structure and have increased efficiency due to very small rotor losses.
There is the necessity for a mechanical transmitter for detecting the magnet wheel position for executing control algorithms in a dynamically high-quality, field-oriented or magnet wheel-oriented control method. It is therefore the goal of many research activities to replace the mechanical transmitter by mathematical models or by utilizing physical effects.
Various methods for determining the position of a magnet wheel of a permanent magnet-excited synchronous machine are known.
Such a method is described in "Algorithms zur rechnerischen Erfassung der Polradlage einer permanentmagneterregten Synchronmaschine ohne Lagegeber" ["Algorithms for Calculated Detection of the Magnet Wheel Position of a Permanent Magnet-Excited Synchronous Machine without a Position Transmitter"] by M. Schroedl and T. Stefan, in the Proceedings (pp. 48 to 54) of the ETG/VDE Conference "Antriebssysteme fur die Gerate- und Kraftfahrzeugtechnik" ["Propulsion Systems for Apparatus and Motor Vehicle Technology"], held in 1988 in Bad Nauheim, FRG. There, the detection of the magnet wheel position of a full pole machine is performed by evaluating the induced voltage. Starting at a certain mechanical revolutions per minute (rpm), a permanent magnet-excited rotor can be used as a transmitter, because a voltage area indicator induced in a stator coil is generally connected with the wanted rotor position in an unambiguous way. In this case, it is permissible to allow non-sinusoidal induction distribution in the air gap. This induced voltage area indicator can be calculated from the terminal voltages, taking into consideration the ohmic and inductive voltage drops.
In this respect, it is disadvantageous that this evaluation can only take place starting from defined minimum rpm, because the amount of the induced voltage area indicator decreases proportionally with the rpm.
The chapter "Detection of the Rotor Position of a Permanent Magnet Synchronous Machine at Standstill" by M. Schr/e,uml/o/ dl, contained in the Proceedings, published in connection with the International Conference on Electrical Machines in Pisa, Italy, in 1986, reports another method.
In this method, the varied magnetic saturation caused by the permanent magnets is measured by means of electrical measurement signals. Because this type of measurement is reproducible, the rotor position can be exactly determined. The knowledge of the polarity of the magnets required for executing this measurement can be obtained by changing the magnetic operating point and measuring its effect on the impedance. In this case, the rotor position can also be determined with the machine stopped.
The disadvantage of this method lies in that the measuring process becomes very expensive because of the requirement for an additional analog current source.
The dissertation of H. Vogelmann, "Die permanenterregte umrichtergespeiste Synchronmaschine ohne Polradgeber als drehzahlgeregelter Antrieb" ["A Permanently Excited, Frequency Converter-Supplied Synchronous Machine without a Magnetic Wheel Transmitter as an Rpm-Controlled Drive"] (Karlsruhe University, FRG, 1986) also deals with a method for detecting the magnet wheel position.
In this case, a relatively high-frequency current, generated by means of the frequency converter, is superimposed as a test current on the actual useful signal. The basic idea here is that an electrical alternating signal connected in a defined (area indicator) direction in general also causes a reaction in the orthogonal direction because of the various inductivities in the longitudinal and crosswise axes. Such coupling does not occur only in the case where the alternating signal is applied exactly in the longitudinal or crosswise direction of the rotor. This results in a criterion, showing whether the signal is applied in the wanted defined direction or not. A prerequisite for obtaining exact measurement results is a permanent magnet-excited synchronous machine, having a salient pole characteristic, i.e., with dissimilar inductivities in the longitudinal and crosswise directions, such as with flux-concentrating devices.
The majority of permanent magnet-excited synchronous machines, however, are not produced as flux-concentrating types, but with a constant air gap and magnets glued on the rotor surface. This is a simpler construction technique and permits air gaps of approximately 1 Tesla, when high-quality samarium-cobalt or neodymium-iron magnets are used.
Thus, there is the disadvantage in the above-mentioned detection methods in that useful results are only obtained with machines having definite salient pole characteristics.